Transmissions convert rotary motion of a drive shaft to rotary motion of a driven shaft at a different speed and torque. Mechanical transmissions typically make use of gearing having two or more selectable fixed ratios to reduce or increase rotation of an input shaft. Hydraulic transmissions make use of fluid interacting with impellers to transmit rotary motion from one shaft to another over a range of drive ratios. Both have drawbacks.
Gearing typically operates using fixed input to output ratios. The ratio can usually be changed only by changing gears. Such transmissions are therefore usually large and heavy mechanisms. Weight of the transmission and friction loss through the various combinations of gears can substantially reduce the efficiency of an entire drive train.
The disadvantage of hydraulic conversion is the ever-present "slip" from the lack of positive interaction between driving and driven shafts. Hydraulic transmissions also suffer from bulk and are typically very costly.
Many attempts have been made to eliminate these disadvantages by providing a continuously variable speed change by simple mechanical means. However, a need remains for an infinitely variable speed transmission which is reliable and capable of a long operating life.